“During the past few days, former President Nelson Mandela has had a recurrence of lung infection,” said the statement by President Jacob Zuma. “This morning at about 1:30 a.m. [7:30 p.m. Friday ET] his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition."

“The former president is receiving expert medical care and doctors are doing everything possible to make him better and comfortable,” it added.

A spokesman for the South African presidency, Mac Maharaj, told Sky News that Mandela was "able to breathe on his own," adding "I think that's important."

Maharaj said he was trying not to paint "a rosy picture" but added "neither do I want to paint a bad picture and say 'let's give up."

"He's a fighter. He's been through this many times, he's been through worse issues and he has survived," Maharaj said.

Asked about Mandela's general state of health, the spokesman said "there are moments when he looks terribly frail, within minutes he changes so you think he is completely normal and usual."

The Nobel Peace Prize winner and longtime political prisoner led the nation’s battle against the white-minority apartheid government.

He became the first president of the country to be elected following the fall of the apartheid system, leading his country as head of the African National Congress from 1994 to 1999.

Mandela has a history of lung problems dating back to when he contracted tuberculosis while being held in prison under the apartheid regime.

He has been in and out of the hospital several times in recent months, most recently being discharged from a hospital on April 6 after having been treated since March 27 for pneumonia and other problems.

Zuma’s ANC government was recently criticized for allowing a visit by the president and other party leaders with Mandela to be broadcast on national television because of his ailing health.

The use of the word “serious” to describe Mandela’s condition was a cause of concern for ordinary South Africans.

"It's such painful news but I pray for him that he can get better and better and better as he is the best man in this country," Pretoria resident Khodani Mulwena told Reuters.

"He is going to survive," Willie Mokoena, a gardener in Johannesburg, told The Associated Press. "He's a strong man."

Concern for Mandela's health united the ANC and the opposition.

“We will keep President Mandela and his family in our thoughts and prayers at this time and call upon South Africans and the peoples of the globe to do the same for our beloved statesman and icon, Madiba,” the ANC said in a statement.

“Nelson Mandela is a father to South Africa and South Africans; every time he is admitted to hospital we feel saddened along with the rest of our country,” the Democratic Alliance, the main political opposition party, said in a statement.